Deep penetrating nevus (DPN) is a variant of melanocytic nevus which goes unrecognized due to its relative rarity and may be misinterpreted as malignant melanoma. It commonly presents in young adults as a dark pigmented lesion on the face, neck, or shoulder. A 60-year-old lady presented with a mole over the left arm of 8 years duration. A biopsy of the lesion was performed under the clinical impression of a compound nevus with suspicion of malignancy. Based on the histologic features, a diagnosis of DPN was put forward.

Deep penetrating nevus (DPN) is a pigmented lesion that clinically and histologically can mimic a malignant melanoma. It occurs predominantly in head and neck and upper extremities of young individuals. DPN needs to be distinguished from malignant melanoma and also from other benign lesions as blue nevus, cellular blue nevus, spindle and epithelioid nevus, pigmented spindle cell nevus and congenital melanocytic nevus. We present a case of DPN in a 60-year-old lady, describe its histopathologic features and provide a brief review of the literature.

Case Report

A 60-year-old female presented to the skin department with mole of 8 years duration over the left arm. She also complained of associated burning sensation for 3 years which gradually increased. The patient was a known diabetic on treatment for 10 years. There was no other significant past history. On examination, a single, pigmented papule with rough surface measuring 1 × 0.5 cm was present over the left arm. The surrounding area showed induration. A clinical diagnosis of compound nevus was given and excised lesion was sent to histopathology to rule out malignancy.

Morphology

The specimen we received consisted of single skin covered tissue measuring 1 × 0.5 cm. Microscopy revealed a pigmented, pyramidal-shaped lesion in the dermis with its base abutting towards the epidermis and apex extending deep into the reticular dermis. The nevus cells were large epithelioid to spindly. They were arranged loosely in nests and fascicles and showed nuclear pleomorphism and hyperchromasia [Figure 1]. Few cells showed eosinophilic nucleoli and nuclear pseudoinclusions [Figure 2]. The cells were seen surrounding the skin adnexa. Interspersed melanophages and multinucleated giant cells were seen along with occasional mitoses and lymphocytic infiltrate. A histopathological diagnosis of DPN was given.

DPN was first described by Seab et al. in 1989. [1] The most frequent clinical presentation is that of a deeply pigmented papule on the face, head, neck, trunk, or proximal extremity of a young to middle-aged adult. [2] The size can range from 2 mm to 1 cm. It is nearly always acquired and somewhat more common in females. [3]

Clinically, DPN can be misinterpreted as a number of different entities including blue nevus, seborrheic keratosis, tattoo, haemangioma, enlarging mole, and pigmented Spitz nevus. [4] The most important differential diagnosis is that of thick melanoma. [5] However, histologically, DPN has a distinct morphology characterized by a wedge-shaped or plexiform growth pattern composed of deeply pigmented spindle and epithelioid cells often extending into the subcutis. Higher magnification shows clustering of pigmented nevus cells that contain pleomorphic, vacuolated nuclei with smudged chromatin. Mitosis is absent and scattered melanophages and little or no inflammatory reaction are seen. Nuclear vacuoles and pseudoinclusions can often be observed. [1] Barnhill described a case of plexiform spindle cell nevus which had features similar to DPN. [6] The plexiform spindle cell nevus has now been considered as DPN. [6]

DPN can be falsely diagnosed as malignant melanoma (MM). [1] However, its architecture is different from malignant melanoma. The neoplastic melanocytes of malignant melanoma originate at the dermoepidermal junction and invade the dermis in an irregular, destructive way. [1] In contrast, the junctional component of most lesions of DPN is inconspicuous and dermal collagen and adnexal structures are undisturbed. Other features that favour the diagnosis of malignant melanoma include mitosis, atypical melanocytes with hyperchromatic nuclei, coarse granular chromatin, and epithelioid cells with fine pigment, solid growth pattern, reactive stroma and plasma cells. The other microscopic differential diagnoses of DPN include blue nevus, cellular blue nevus, spindle and epithelioid cell nevus, and congenital melanocytic nevus. Lesions of blue nevus can be differentiated by their characteristic dendritic melanocytes and absence of junctional activity. [1] Lesions of cellular blue nevus are characterized by prominent nests of nonpigmented or sparsely pigmented spindle cells and absence of junctional nests. [1] Lesions of spindle and epithelioid cell nevus can be distinguished from DPN by their conspicuous junctional nests, epidermal acanthosis and vascular telangiectases of papillary dermis. [1] Congenital melanocytic nevi are present since birth and there is no nuclear atypia. [7]

On immunohistochemical analysis, DPN shows positivity for both S-100 and HMB-45 and negativity for keratins. Hence, immunohistochemistry is of no value in differentiating this entity from malignant melanoma. [4]

Excision is the treatment of choice. [4] DPN does not have propensity to recur and no further treatment is recommended as long as it is completely excised.

To conclude, DPN is a distinct entity with characteristic architectural and cytologic features which remains a diagnostic challenge to pathologists. Accurate diagnosis of this entity is important as it has a favourable prognosis and treatment course.